This invention relates to an integrated, improved self-steering lift axle wheel assembly of the type used to increase the capacity, performance and safety of load-carrying vehicles, wherein the lift axle wheel assembly may be moved downward, contacting the surface of the road when the vehicle is heavily loaded, thereby sharing the load. Alternatively, when the vehicle is not heavily loaded the lift axle wheel assembly may be lifted upward, away from the surface of the road, thereby reducing friction. Some of the benefits offered by the improvements disclosed herein include:
(1) a significant reduction in the weight of the assembly which translates into increased fuel efficiency and payload capacity for vehicles so equipped; and
(2) torque and stresses caused by irregularities in road surfaces are absorbed by the composition and placement of the components, and not transferred into the vehicle frame and driver, thereby resulting in less maintenance on the vehicle and increased driver comfort.
Lift axle wheel assemblies have been conventionally used by the trucking industry to respond to and comply with the various state regulations for limiting the maximum gross weights that trucks may legally carry on interstate roads. The weight of a loaded truck is transferred onto the available axles and wheels that support the vehicle. Calculations of gross weight are generally referenced to a maximum weight per axle. If a truck is equipped with a lift axle wheel assembly and desires to carry a load that would exceed the legal or safe limits imposed by the fixed axle configuration of the vehicle, the lift axle wheel assembly may be placed in its downward position, thereby transferring a portion of the load borne by the fixed axles and wheels onto the lift axle wheel assembly. The vehicle may thereby comply with the applicable regulations while still carrying the greater load that would otherwise require a second trip or a second vehicle. There is also a corresponding decrease in damage to road surfaces because of the more equal distribution of weight.
There are other important benefits that make the availability of a lift axle wheel assembly desirable. Because an assembly of this type may be retracted into its upward position when extra load-carrying capacity is not needed, the decrease in fuel consumption caused by the increased friction that would otherwise occur with an increase in the number of tires contacting the surface of a road is limited to those instances when more carrying capacity is essential and the lift axle wheel assembly is extended downward in its engaged position. A single vehicle may thus haul loads with different weights and still function with fuel efficiency. Moreover, wear on the tires associated with the lift axle wheel is limited to just those periods when the assembly is in use. Other ancillary, yet important benefits are increased driver comfort and vehicle control. Stresses that are produced by road surface irregularities are absorbed by the lift axle wheel assembly, not the driver or by vehicle equipment. Thus, driving is made less physically demanding and tiring. Because the stresses that the vehicle are subject to are more widely dispersed and distributed, vehicles equipped with a lift axle wheel assembly tend to maintain a longer useful life and require less frequent maintenance, and are not as prone to "shimmy" due to resonant frequency effects caused by road surfaces.
As mentioned earlier, the use of auxiliary lift axle wheel assemblies for the reasons stated above is well known. The prior art includes lift axle wheel assemblies of various designs whereby inflatable air bags are used to force the assembly into a downward position relative to the vehicle frame, causing the tires on the assembly to contact the road surface, and thereby redistributing the weight of the load and adding to the gross vehicle weight. Mounting of the air bags has been made directly to the axle and frame, as shown in U.S. Pat. No. 3,730,549. Another method of mounting is shown in U.S. Pat. No. 3,771,812 in which the lift wheel assembly is supported at its ends by pivotable levers whose other ends are mounted to the vehicle frame, and inflatable air bags are mounted to the respective levers midway between the ends of the pivotable levers. In both of the aforedescribed prior art configurations, the introduction of air into the air bags forces the assembly downward, away from the frame and into contact with the road surface.
Systems to return or maintain the lift axle wheel assembly in its retracted position, up and out of contact with the surface of the road, have generally been based on the use of springs, a second pair of inflatable air bags, or a combination of springs and a second pair of air bags. U.S. Pat. No. 3,772,812 shows the use of a coil tension spring that is expanded by the downward motion of the lift axle assembly caused by the introduction of air into the air bags. When the air in the air bags is exhausted, the springs, which are normally biased in a closed position, return the lift axle wheel assembly into an up position. Leaf springs have been used in place of coil tension springs to accomplish the same result. In a lift axle wheel assembly that uses a pivotable lever mounting, it is possible to design the levers in such a manner that inflatable air bags are mounted on lever ends which are opposite the ends that attach to the lift axle wheel assembly. The assembly is raised into an upward position by the forcing of air into this pair of air bags, while the exhaustion of air therefrom forces the lift axle wheel assembly into its downward, engaged position.